1. Field of the Invention
The present invention relates to a method of producing an epitaxial silicon wafer, specifically a method of producing an epitaxial silicon wafer in which an epitaxial film is vapor-deposited on a mirror-polished front surface of a silicon wafer.
2. Description of Related Art
An example of substrates for fabrication of bipolar IC devices is an epitaxial silicon wafer. Such an epitaxial silicon wafer is produced by vapor-depositing an n-type epitaxial film composed of monocrystalline silicon having a thickness of several μm on a p-type silicon wafer having low resistance (approximately 0.01 Ω·cm).
A silicon wafer having a mirror-polished front surface is produced by slicing a monocrystalline silicon ingot grown in the Czochralski (CZ) process; chamfering, lapping (grinding), and etching the sliced silicon wafer in the sequence; and then polishing the front surface of the wafer.
In a general polishing method, the front surface of the silicon wafer undergoes polishing steps including primary polishing, secondary polishing, and finish polishing in the sequence, each of which is followed by cleaning. The wafer front surface is treated through the multiple polishing steps such that, for example, polishing abrasive grains become finer and a polishing cloth becomes less hardened as the polishing proceeds to the next step so as to achieve low surface roughness of the wafer surface.
In the precision polishing method that involves such multiple steps, however, a polishing time is extended for a low-resistance wafer having a higher hardness due to repeated polishing and cleaning through the steps. Accordingly, the flatness of the front surface of the silicon wafer is degraded, thus causing pits in the wafer front surface and sagging or periodic unevenness in the external peripheral portion of the silicon wafer. Furthermore, cost is another issue in the multiple-step polishing of the silicon wafer, including primary polishing, secondary polishing, and finish polishing.
In view of such circumstances, a conventional technology is disclosed in Related Art 1, for instance. In the technology, a front surface of an etched silicon wafer undergoes only primary mirror-polishing, and then an epitaxial film is vapor-deposited on the polished surface. Polishing liquid including free abrasive grains, such as, for example, colloidal silica, is used in the primary mirror-polishing. The roughness of the primary mirror-polished surface is 0.3 nm or greater and 1.2 nm or less in RMS (root mean square) in measurement of a measurement area of 1 μm×1 μm using an atomic force microscope.
[Related Art 1] Japanese Patent Publication No. 3120825
However, the polishing liquid used in the primary mirror-polishing in Related Art 1 includes free abrasive grains. Thus, the surface roughness of the front surface of the silicon wafer after primary mirror-polishing is 0.3 nm or greater in RMS in measurement of the measurement area of 1 μm×1 μm using the atomic force microscope. This value is not deemed sufficient to meet rapid high integration of devices in these days. In addition, the RMS value is extremely high in a large measurement area of 10 μm×10 μm, and a front surface of an epitaxial film thereafter is also extremely rough.
In the case where the wafer front surface undergoes primary mirror-polishing with the polishing liquid including free abrasive grains as shown in Related Art 1, new process damage is introduced in a wafer front layer portion by mechanical action during mirror-polishing. Thus, numerous process damages having a depth of 5 nm or greater in general are caused in the primary mirror-polished surface, adversely affecting the gate oxide integrity. Furthermore, defects attributable to processing, such as micro scratches, are caused in the primary mirror-polished surface of the silicon wafer because of aggregation of free abrasive grains in the polishing liquid. Such defects lead to a number of LPDs (light point defects) in the primary mirror-polished surface. Depositing an epitaxial film on such a wafer front surface increases the density of LPDs observed in the front surface of the epitaxial film. Specifically, more than 1,000 LPDs having a size of 130 nm or greater appear in a single silicon wafer having a diameter of 300 mm.
As a result of intense research, the inventors have found that all circumstances above are solved by mirror-polishing a front surface of a silicon wafer using polishing liquid that includes water-soluble polymers but not free abrasive grains and then by vapor-depositing an epitaxial film on the wafer front surface.
The present invention provides a method of producing an epitaxial silicon wafer provided with an epitaxial film having low LPD density and low surface roughness by reducing the density of LPDs attributable to processing and caused in a mirror-polished front surface of a silicon wafer and by reducing the surface roughness of the wafer front surface.